This investigation is for the purpose of determining biological relationships, if any, between the nucleosides, particularly adenosine, 6 methyl adenosine, and 5 methyl cytidine, and a transmissible entity which has been discovered in females of the C3H/St strain and has also been proven to be an inhibitor of spontaneous tumors in mice. The points of comparison are measured for several characteristics of spontaneous tumors of mammary gland origin. These characteristics are: (1) growth rates of tumors, (2) regressions of tumors with or without recurrence, (3) percentages of mice showing multiple primary tumors and (4) survival times of mice bearing spontaneous tumors. The nucleosides are used separately or in combinations.